(1) Field of the Invention
The present invention relates to an automatic focusing apparatus. More specifically, the present invention relates to an automatic focusing apparatus in devices such as a pick-up device employed for recording and reproducing information into and from an optical disk.
(2) Description of the Prior Art
The conventional automatic focusing apparatus in a pick-up device or the like used in an optical disk device, employs the astigmatism method or the light-blocking method. In the former method, astigmatism is caused to occur by the use of a cylindrical lens or the like and a focusing error signal is obtained by the use of a four-element photo detector. On the other hand, in the latter method, a portion of reflected light is blocked by means of a knife edge or the like and an image of a plane including this knife edge is formed via a converging lens on a four-element detector thereby obtaining a focusing error signal.
In these methods, since the cylindrical lens, the knife edge or the like is employed in the focusing detecting optical system as noted above, if it is attempted to increase sensitivity of the automatic focusing apparatus, the cylindrical lens needs to have a longer focal length and to be placed an adequately long distance from the photocell. As the result, the whole apparatus tends to be large and heavy.